Red light =60/3 = 20 sec and green light = 5 = 24 sec. They will flash together in 120 sec (LCM of 20 and 24); i.e. 2 min. 60 No. of times they flash in an hour = 1/2 = 30

Solution:

A check may be carried out as follows:

Say, we have 500 ml of alcohol in beaker 1and 500 ml of water in beaker 2. 

Now, assume 3 cups hold 300 ml of any liquid — alcohol or water, so if we take 3 cups of alcohol from beaker 1 (the cup will have 300 ml of alcohol) and pour it into beaker 2…

In Beaker 1:

200 ml of alcohol is left

In Beaker 2:

…we will have 500 ml of water and 300 ml of alcohol as a mixture, 800 ml mixture.

In this mixture, alcohol is 300/800 parts and water is 500/800 parts.

Now, if we fill 3 cups from beaker 2…

In Beaker 2 :

500 ml mixture remains, the constituents of the mixture are:

{3/8}*500 =187.5ml alcohol and{5/8}*500=312.5  ml water

In Beaker 1:

We were holding 200 ml of alcohol to which 300 ml of mixture is added from beaker 2.

The constituents of 300 ml of mixture that is added are:

{3/8}*300=112.5ml of and {5/8}*300ml water

Therefore, final situation of constituents in Beaker 1 = 200 + 112.5 = 312.5 ml alcohol

…and187.5 ml water

Comparison:

Percentage of water in beaker 1 = {187.5/500}*100

Percentage of alcohol in beaker 2 = {  187.5/500}*100

Solution:

30 litres of the mixture has milk and water in the ratio 7 : 3. i.e. the solution has 21 litres of milk and 9 litres of water.

When you add more water, the amount of milk in the mixture remains constant at 21 litres. In the first case, before addition of further water, 21 litres of milk accounts for 70% by volume. After water is added, the new mixture contains 60% milk and 40% water.

Therefore, the 21 litres of milk accounts for 60% by volume.

Hence, 100% volume = 21/0.6=35litres.

We started with 30 litres and ended up with 35 litres. Therefore, 5 litres of water was added.

Solution:

1/A=10 and 1/B+1/C=1/4

 (Given)

1/A+1/B+1/C=1/4+1/10=7/20

In 20 hours, working together they will complete 7 products. 

Thus in 40 hours they will complete 14 products